It is proposed that energy harvesting is utilized in a sensing terminal (a terminal in which a wireless module and a sensor are combined) that has attracted rising attention in recent years, and so forth.
Electric power generated by an energy harvesting element (for example, a photovoltaic cell or the like) is stored in a power storage element such as a secondary battery in order to supply stable electric power to a load circuit of a sensing terminal. However, if environmental energy becomes weak, there is a possibility that an output voltage of the energy-harvesting element becomes lower than a voltage of the power storage element and a reverse current, headed from the power storage element to the energy-harvesting element, occurs, thereby consuming electric power.
In the past, in order to prevent such a reverse current from occurring, there has been a technology for periodically comparing a voltage generated by a photovoltaic cell and a voltage of a secondary battery with each other by using a comparison circuit, thereby performing on-off control of a switch between the photovoltaic cell and the secondary battery. In addition, there has been a technology for controlling a switch disposed between a photovoltaic cell and a secondary battery, based on a result of a comparison between a potential difference between both terminals of a reverse-current preventing diode (a potential difference between both terminals of the switch) and a predetermined value, thereby preventing a reverse current from occurring, the comparison being made by using a comparison circuit, the reverse-current preventing diode being disposed in parallel with the switch.
However, in the technology for periodically comparing the generated voltage and the voltage of the secondary battery, even if the generated voltage is greater than the voltage of the secondary battery at comparison timings, there is a possibility that the switch remains turned on, thereby causing a reverse current, in a case where the generated voltage is reduced between the comparison timings. In addition, in a case of applying a photovoltaic cell to, for example, an Internet of Things (IoT) apparatus used in a room, a generated current becomes low. Therefore, in the technology for detecting a potential difference between both terminals of the switch, in a case where on-resistance of the switch is low, the potential difference becomes very small, and it is difficult to detect the potential difference by using the comparison circuit.
In this way, in the conventional technologies, there is a problem that it is difficult to suppress the occurrence of the reverse current.
The followings are reference documents.    [Document 1] Japanese Patent No. 3271992,    [Document 2] Japanese Laid-open Patent Publication No. 09-261861,    [Document 3] Japanese Laid-open Patent Publication No. 2000-347753, and    [Document 4] Japanese Laid-open Patent Publication No. 04-265639.